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Metals - Extraction of Aluminum (Bauxite)

Grade 11A LevelChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Aluminum is extracted from its primary ore, bauxite, which is purified to form alumina (Al2O3Al_2O_3).

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Because aluminum is more reactive than carbon, it cannot be reduced by carbon heating; instead, electrolysis of the molten oxide is required.

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The melting point of pure Al2O3Al_2O_3 is very high (over 2000∘C2000^{\circ}C). It is dissolved in molten cryolite (Na3AlF6Na_3AlF_6) to lower the melting point to approximately 950∘C950^{\circ}C, which saves energy and costs.

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Cryolite also improves the electrical conductivity of the electrolyte.

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The electrolysis cell uses carbon (graphite) for both the anode (positive electrode) and the cathode (negative electrode).

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At the cathode, aluminum ions (Al3+Al^{3+}) undergo reduction to form liquid aluminum metal: Al3++3e−→Al(l)Al^{3+} + 3e^{-} \rightarrow Al(l).

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At the anode, oxide ions (O2−O^{2-}) undergo oxidation to form oxygen gas: 2O2−→O2(g)+4e−2O^{2-} \rightarrow O_2(g) + 4e^{-}.

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The oxygen gas produced at the anode reacts with the graphite electrodes at high temperatures to produce carbon dioxide gas (C+O2→CO2C + O_2 \rightarrow CO_2), meaning the anodes wear away and must be replaced regularly.

📐Formulae

Al3++3e−→AlAl^{3+} + 3e^{-} \rightarrow Al

2O2−→O2+4e−2O^{2-} \rightarrow O_2 + 4e^{-}

2Al2O3→4Al+3O22Al_2O_3 \rightarrow 4Al + 3O_2

C(s)+O2(g)→CO2(g)C(s) + O_2(g) \rightarrow CO_2(g)

💡Examples

Problem 1:

Explain why the carbon anodes in the extraction of aluminum must be replaced periodically.

Solution:

The oxygen gas produced at the anode reacts with the hot graphite (carbon) to form carbon dioxide gas: C(s)+O2(g)→CO2(g)C(s) + O_2(g) \rightarrow CO_2(g).

Explanation:

Because the anode is made of carbon and the process occurs at a high temperature, the byproduct oxygen chemically attacks the electrode, causing it to burn away over time.

Problem 2:

State two reasons why cryolite is used in the electrolysis of aluminum oxide.

Solution:

  1. To lower the melting point of the electrolyte from 2000∘C2000^{\circ}C to 950∘C950^{\circ}C. 2. To increase the electrical conductivity of the mixture.

Explanation:

Using cryolite reduces the thermal energy required, making the process commercially viable and providing a better medium for ion movement.